1. Field of the Invention
The invention relates to plastic articles or parts that exhibit low friction and good wear resistance. The invention more particularly relates to an improved piston for disc brakes, such as vehicular disc brakes, particularly automotive disc brakes. The present invention relates to an improved plastic brake piston for use in anti-skid or anti-lock vehicular brake systems, now commonly known as the "ABS" system.
Although the present invention will be described in terms of disc brakes, particularly intended for motor vehicles, it will be understood that the present improved piston is equally adaptable to piston brakes utilized on aircraft and railroad vehicles and other hydraulic or air-activated pistons.
One type of brake assembly presently in use in motor vehicle disc brakes is the opposed piston type. In such type, a piston is positioned adjacent each opposite side of a brake disc and forces its respective brake shoe or lining against the disc to impart a braking action. Other types of brake assemblies in which the present invention is equally useful are those in which a single piston is utilized to move the brake shoe or lining into contact with both sides of the brake disc by utilizing a caliper slide to impart a braking action.
Upon the advent of the use of disc brakes in motor vehicles, the pistons used were metallic, usually fabricated of chrome plated iron or steel, and required several time consuming and costly machining operations. More recently, pistons for disc brakes have been fabricated of a plastic material, usually a phenolic resin molding compound. Such pistons may be produced by molding techniques known in the art. The molded plastic pistons are generally machined to achieve close dimensional tolerance.
2. Prior Art
Plastic pistons are described in U.S. Pat. No. 4,170,926, the disclosure of which is incorporated herein by reference.
Additional modified plastic pistons are disclosed in U.S. Pat. No. 4,401,012, the disclosure of which is incorporated herein by reference.
The plastic pistons made in accordance with the above-disclosed patents are very serviceable in conventional brake systems.
However, in the new ABS braking systems, brake manufacturers have seen wear on the outside surfaces of the pistons when the pistons are subjected to static testing. Piston wear occurs where the piston is in direct contact and slides against the steel or aluminum caliper bore. Steel pistons also have been observed to develop wear marks.
It is a purpose of this invention to reduce friction or eliminate the wear on plastic articles and parts, for example, on the outside walls of the plastic brake piston, particularly those made from phenolic molding compounds.
U.S. Pat. No. 3,992,221 discloses treating the surface of an extensible hydrocarbon article, such as a rubber glove, by positioning the article in a chamber from which extraneous oxidizing agents are excluded, extending the article, and treating the extended surface with a fluorinating gas which may contain elemental fluorine. The treatment of the non-filled elastomer provides improved frictional characteristics, chemical inertness, and other desirable properties.